1. Field of the Invention
This invention relates to the field of seating systems and more particularly to the field of seating systems for wheelchairs.
2. Discussion of the Background
It is known and well documented that persons confined for extended periods of time to seating systems (particularly seating systems for wheelchairs) are subject to and at risk of developing decubitus ulcers or pressure sores. These pressure sores normally occur in the tissue surrounding or adjacent to the bony prominences of the skeletal structure and are due to the pressure applied to them by the weight of the seated person. That is, the tissue between the seated person's bones and the seating surface is normally squeezed under the user's weight. As a result, the normal flow of blood through the capillary vessels to the squeezed tissue areas can become occluded or blocked. Continued pressure and the resulting lack of blood flow to the tissue will cause necrosis (dead tissue) to form and in the most severe cases may cause an open wound from the epidermis (skin) all the way down through the fatty and muscular tissue to an exposed bone. Unattended, the decubitus ulcers (pressure sores, wounds) may become infected and eventually may even become fatal to the seated person. The most common areas of pressure sore development are the ischial tuberosities and the coccyx (tailbone).
There have been numerous prior art approaches developed to specifically deal with the problems of users confined to seating systems over long periods of time such as wheelchair users. One such prior art approach is disclosed in the present assignee's U.S. Pat. No. 4,588,229. This approach has proven to be effective in reducing the risk of pressure sores or ulcers as well as providing stable and comfortable positioning for the user. In it, a seating system is provided consisting essentially of a relatively rigid, shaped tray or base member and a fluid pad member. The tray or base member is contoured to provide a seating well to accept the bulk of the fluid pad. The fluid pad is designed to conform to the precise shape of the seated user and in doing so, the interface pressures where the fluid pad contacts the user are significantly reduced to safe levels. The bulk of the fluid pad is concentrated about the high risk, boney prominences (i.e., ischial tuberosities, coccyx, and trochanters). In these areas, it is desirable to achieve as much surface contact as possible between the buttocks and the fluid paid to thereby lower the average interface pressure.
This goal of low interface pressure is achieved in such prior art approaches by optimizing the amount of depth immersion of the user into the fluid pad. For example, when only small amounts of depth immersion are occurring (e.g., with a person having a relatively flat buttocks with minimal boney protuberances or with an overfilled pad), the solution is usually to remove some of the fluid from the pad to allow the user's buttocks to sink farther into the fluid pad. This increases the surface contact area and, in turn, favorably reduces the average interface pressure. Conversely, if there is too much depth immersion and the user bottoms out or nearly bottoms out on the rigid tray, pressure sores or ulcers may develop. In such cases, fluid is usually added to the fluid pad to safely support the user's buttocks on a layer of fluid instead of the rigid tray.
Proper sizing of the tray and fluid fill of the pad in such prior art approaches is critical in determining whether the user will bottom out or otherwise fail to achieve optimum pressure relief. The size of such trays should approximately match the seated width of the user. The amount of fluid fill of the pad has largely been determined to date by trial and error using essentially a standard sized person for a given seated width. The nominal fill amount so determined then adequately serves the majority of such users. However, when bottoming out is observed, corrective action must be taken such as adding more fluid to the pad or placing a discrete pouch or pouches of fluid between the fluid pad and the rigid tray. Unfortunately, this can become a fitting nightmare requiring costly and time-consuming visits to a physical therapist and customized, special orders and possibly re-orders. Even then, the optimum pressure relief may still not be achieved: or, a user may gain or lose weight and buttocks mass and a previously properly fitted pad may become ill fitting. Further, the user may have little or no feeling in his buttocks and may not know his seating system is improperly fitted until dangerous decubitus ulcers begin to develop.
The basic problem with all known prior art approaches including liquid and air filled ones is that there is no way to fully anticipate the exact size, weight, and shape of the user's buttocks. Consequently, some current seating systems simply provide a standard fill (or progressions of standard fills) that will prevent bottoming out for the majority of users. Other current seating systems require that the user himself make adjustments to the working fluid (e.g., adding liquid or pumping exhausting air). The first-mentioned systems are simply a compromise and in many cases will not optimize the pressure relief. The second approach may introduce and usually does introduce adjustment errors that may result in less than optimized pressure relief.
With the above in mind, the self-adjusting seating system of the present invention was developed. With it, pressure relief can be optimized automatically without any fill or other adjustments required.